I have original iPad and was told to download ios 5.0 and cannot. Why?

if you have laptop/desktop computer you can sync the iPad with iTunes and download iOS 8.1.2 there:
Upgrading iPad to iOS 8: Through iTunes 1. Backup your iPad
Open iTunes. Select your iPad in iTunes and click Back Up Now.
2. Restart your iPad and your Computer
3. Download and Install the Update
Go to iTunes -> Select iPad -> Check for Update -> Click Download and Update

Whichever method you choose, the process will take quite some time to work depending on how much free storage you have on your iPad, your internet connection, and how Apple's servers are doing with upgrades. Occasionally, Apple will either not let you download the upgrade or it will fail. For iOS 8, you must have an iPad 3 or newer. This does not work with the original iPad - if something goes wrong, restart your device, and try again.

How to Upgrade to iOS 7 on an iPad or iPad Mini
How to Install iOS 8 When Your Device is Low on Storage

To be able to install Viber, your iPad needs at least iOS 6.0 Apple did not release iOS6 for the iPad 1. So your is running on iOS 4 or 5.

For the iPad first generation (A1219) before iOS 5 to update is only possible with connecting the device to iTunes on a computer. Plug it into your computer and open up itunes under devices click on your ipad and then click on check for update.
- but this model supports only iOS 5
- NO iOS 6/7/8 !

iOS cannot be downloaded with safari
There are two ways to update the iOS version of your iPad 2:
- via connecting the device to iTunes on a computer
- or directly from within the device
but only since iOS 5 it is possible to update the iOS version without the need to connect the iOS device to a computer (over-the-air update)
just go to: settings - general - software update

no, you cannot.
on a 1.gen. pad it is impossible to install iOS6:
this model supports only iOS 5 !
- NO iOS 6/7/8
- the processor is too slow....
For the iPad first generation (A1219) before iOS 5 to update is only possible with connecting the device to iTunes on a computer. Plug it into your computer and open up itunes under devices click on your ipad and then click on check for update.
- but this model supports only iOS 5

iPad Wi Fi Original 1st Gen 16 32 64 GB Specs iPad Original A1219 MB292LL...

- this model supports only iOS 5 !
- NO iOS 6/7/8 !
-----------------------------------------------------
For the iPad first generation (A1219) before iOS 5 to update is only possible with connecting the device to iTunes on a computer. Plug it into your computer and open up itunes under devices click on your ipad and then click on check for update.

on a iPad 1.gen. iOS 6 is impossible:
this model supports only iOS 5 !
- NO iOS 6/7/8
- the processor is too slow....
For the iPad first generation (A1219) before iOS 5 to update is only possible with connecting the device to iTunes on a computer. Plug it into your computer and open up itunes under devices click on your ipad and then click on check for update.
- but this model supports only iOS 5
- NO iOS 6/7/8 !

How to do I update to iOS 5?
depends on the iPad model you have:
For the iPad first generation (A1219) before iOS 5 to update is only possible with connecting the device to iTunes on a computer. Plug it into your computer and open up itunes under devices click on your ipad and then click on check for update.
- but this model supports only iOS 5
- NO iOS 6/7/8 !
-----------------------------//2
for all other models:
There are two ways to upgrade the iOS version of your iPhone, iPad, or iPod Touch - via connecting the device to iTunes on a computer
- or directly from within the device, but only since iOS 5, it is possible to upgrade the iOS version without the need to connect the iOS device to a computer (over-the-air update)
just go to: settings - general - software update

on a 1.gen. iPad it is impossible to install iOS 6 !
this model supports only iOS 5 !
- NO iOS 6/7/8
- the processor is too slow....
For the iPad first generation (A1219) before iOS 5 to update is only possible with connecting the device to iTunes on a computer. Plug it into your computer and open up itunes under devices click on your ipad and then click on check for update.
- but this model supports only iOS 5
- NO iOS 6/7/8 !

The original iPad is only compatible with iOS 5, and can't be upgraded to iOS 6. Time for a new iPad!